1. Field of the Invention
The present invention relates to a separation membrane manufacturing method. More specifically, the present invention relates to a separation membrane manufacturing method capable of easily manufacturing a dense separation membrane.
2. Description of Related Art
In recent years, production of ethanol using biomass technology has drawn attention from the viewpoint of environmental protection and effective utilization of scrap wood. A method of using selective permeability of a zeolite membrane has been known as a method for recovering the ethanol produced by such biomass technology. The method comprises bringing a liquid mixture containing water and ethanol obtained from woody biomass into contact with the zeolite membrane to selectively pass water alone through the membrane, thus separating ethanol from water.
The liquid mixture obtained from woody biomass contains organic acids such as acetic acid along with water and ethanol. A zeolite membrane generally has poor resistance to acids, and thus the organic acids may lower separation performance of the membrane or cause early deterioration of the membrane.
There, a carbon membrane that has been mainly used for separating a particular component from a gas mixture is now studied about the separation of organic solvents such as ethanol from water. A carbon membrane has higher acid resistance than that of a zeolite membrane and provides stable separation performance over a long period of time even in the presence of organic acids. For such a purpose, the carbon membrane is typically used as a carbon membrane provided on the surface of cells formed in a porous monolith substrate.
The method of manufacturing such a separation membrane is exemplified by a carbon membrane manufacturing method as below. First, by passing a separation membrane precursor solution through a plurality of cells formed in a porous monolith substrate, a separation membrane precursor is formed on the surface of the cells. The separation membrane precursor is exemplified by a polyamic acid membrane. Next, the monolith substrate is placed in a dryer, and the separation membrane precursor is dried. Then, the dried separation membrane precursor is thermally decomposed under a reducing atmosphere such as a nitrogen atmosphere to be carbonized, yielding a carbon membrane (for example, see Patent Document 1).
In a separation membrane manufacturing method in Patent Document described above is difficult to uniformly dry the whole membrane, and this adversely affects the separation performance of the resulting separation membrane. Therefore, separation membrane manufacturing methods in which drying of the separation membrane precursor is performed by ventilation drying in which hot air is passed through the cells have been suggested (for example, see Patent Documents 2 and 3). In such a conventional separation membrane manufacturing method, the step of forming a separation membrane precursor and the step of drying the separation membrane precursor are repeated a plurality of times, and consequently a separation membrane having an intended separation performance is manufactured. Hereinafter, the step of forming a separation membrane precursor may be called “membrane forming step”, and the step of drying a separation membrane precursor may be called “drying step”.